1. Field of the Invention
The present invention is generally in the field of printed circuit board packaging. More specifically, the invention is in the field of plastic laminate-based molded integrated circuit (“IC”) packages.
2. Background Art
It is known in the field of plastic laminate-based printed circuit board packaging that plastic laminate-based molded IC packages such as plastic ball grid arrays and multi-chip modules are prone to reliability problems. Electrical failures such as shorts and opens occur in the substrates and at the wire bonds as a result of cracking that occurs in the die attach material used to attach a semiconductor die to the printed circuit board.
These cracks may then propagate into the interface between the mold compound and the laminate material and/or into the mold compound that surrounds the perimeter of the semiconductor die. This cracking is generally referred to as “delamination.” As a result of the temperature changes inherent to the IC package's normal operation, delamination will worsen during the life of the IC package. Thus, over the long term, the failure rate of these IC packages will increase.
One of the main reasons for delamination is the presence of moisture within the mold compound, the resin layers of the printed circuit board, and the die attach material. Such moisture may result from the IC package being subjected to a humid environment.
During subsequent manufacturing processes the printed circuit board may be subjected to heat. For example, during soldering operations the IC package may enter a re-flow furnace where the temperature is well above the boiling point for water. This heat causes the moisture in the die-attachment area (“die-attach area”) to vaporize. This vapor results in upward force in any pockets or voids that may be present underneath the die-attach area. The vapor is unable to pass through the semiconductor die.
Thus, the semiconductor die acts as a barrier to the vapor and there is an upward force on the semiconductor die from the vapor trying to escape into the surrounding environment. This force may cause a separation between the semiconductor die and the die attach material bonding the semiconductor die to the surface of the printed circuit board. In other cases, the semiconductor die will remain attached to the die-attach area but the mold compound will separate from the metal surface as a result of upward force from the vapor.
FIG. 1 shows a cross section of plastic laminate-based molded printed circuit board package 10 after semiconductor die 16 has separated from die-attach area 18. The printed circuit board comprises a layer of solder mask, referred to by numeral 26. A first layer of metal, referred to by numeral 22, rests on top of solder mask 26. A layer of resin, referred to by numeral 24, rests on top of metal 22. A second layer of metal, referred to by numeral 36, rests on top of resin 24. Solder mask 38 covers a portion of metal 36, but does not cover the top area of metal 36 referred to by numeral 40, which is a printed circuit board bonding region. Printed circuit board bonding region 40 is a designated region on the surface of metal 36 used for bonding to bond wires.
Before separation, semiconductor die 16 was attached to die-attach area 18 by die attach material 48, also referred to as die attach 48. Die attach area 18 usually comprises die attach 48 resting on a layer of solder mask which is not shown in any of the Figures in the present application. A bond wire, referred to by numeral 28, is bonded to a die bonding pad, referred to by numeral 14, that rests on top of semiconductor die 16. Bond wire 28 is gold or aluminum. Before separation, the other end of bond wire 28 was bonded to a printed circuit board bonding location referred to by numeral 30, which is located in printed circuit board bonding region 40. Before separation, the mold compound, referred to by numeral 12, completely enclosed metal 36, solder mask 38, semiconductor die 16, bond wire 28, die bonding pad 14 and printed circuit board bonding location 30.
As the printed circuit board is heated, the vapor, referred to generally by numeral 32, results in an upward force, shown as arrows and generally referred to by numeral 34. This upward force, in turn, results in delamination in die-attach 48 and a resulting separation between semiconductor die 16 and die-attach area 18. Semiconductor die 16 in turn forces mold compound 12 to separate from metal 36. Bond wire 28 is detached from printed circuit board bonding location 30 as a result of mold compound 12 separating from metal 36. This results in an electrical open and consequent failure of the circuit.
Mold compounds adhere poorly to the smooth surface of gold. Thus, when metal 36 is gold-plated copper, the delamination problem discussed above is exacerbated by the poor adhesion properties between metal 36 and mold compound 12. As a result of the poor adhesion between mold compound 12 and metal 36, mold compound 12 cannot prevent the separation of semiconductor die 16 from die-attach area 18. Instead, mold compound 12 separates from metal 36 along with semiconductor die 16, as discussed above.
As a result of the problem illustrated by FIG. 1 and discussed above, efforts have been made to improve the strength of the materials used to secure components to the surface of the printed circuit board. In addition, efforts have been made to reduce the moisture absorption properties of mold compounds and die attach materials. Also, there have been efforts to increase the chemical adhesion properties between the mold compounds and metals used in the manufacture of laminate-based molded IC packages.
Because the methods discussed above have not eliminated the delamination problem, efforts have been made to reduce the amount of exposed metal on the surface of the printed circuit board whenever possible. This area can be reduced, for example, by being covered with solder mask 38, since mold compounds adhere better to solder mask than to the exposed metal. Thus, when adhering to solder mask 38, mold compound 12 more effectively hinders the separation of semiconductor die 16 from the surface of the printed circuit board by itself adhering more securely to solder mask 38.
However, in some applications, for example, circuits using Radio Frequency (RF) signals, design engineers require a relatively large area of exposed metal on the surface of the printed circuit board to increase the electrical performance of the circuit. Since this area of exposed metal is very large, it is not desirable to cover it with solder mask. Therefore, the solder mask method discussed above cannot be used effectively.
Thus, there is a need in the art for a cost-effective method to more effectively secure a mold compound on the surface of the printed circuit board.